Work is in progress to measure, by equilibrium ultracentrifugation and other techniques, the dimer-tetramer association constants of hemoglobins A and S under a wide range of conditions. In addition, the gelation behavior of hemoglobin S is being investigated by equilibrium ultracentrifugation. In the next two years, work will be focussed on a study of the carbamyl derivatives of these two hemoglobins. Emphasis is being placed on careful separation of the derivatives under study in order to isolate, as nearly as possible, homogeneous materials for study. Emphasis is also being placed on development of technique, in order to bring to bear as much precision as is presently possible.